1. Field of Invention
The invention relates to a system for displaying an image, and, in particular, to an image displaying system having organic light-emitting devices.
2. Related Art
An organic light-emitting device (OLED) has the advantages of self-emissive property, high luminance, high contrast, light weight, thin profile, low power-consumption and fast response speed. Due to these desirable qualities, it is becoming ever more widely used in various image displaying systems.
The OLED mainly includes a cathode, an electron transporting layer, an organic light-emitting layer, a hole transporting layer and an anode. The organic light-emitting layer is disposed between the electron transporting layer and the hole transporting layer. The cathode provides electrons to be injected into the electron transporting layer. The anode provides holes to be injected into the hole transporting layer. The electrons and the holes are respectively ejected from the electron transporting layer and the hole transporting layer and are recombined in the organic light-emitting layer so that the organic light-emitting layer emits light. Moreover, the OLED often serves as a pixel or a sub-pixel of the image displaying system.
Referring to FIG. 1a, a conventional image displaying system has a plurality of OLEDs, and three OLEDs may constitute a full-color pixel. A pixel 1 includes three OLEDs 11 to 13, and each of the OLED 11 to 13 has a transparent electrode layer 14, a red light organic light-emitting layer 15r, a blue light organic light-emitting layer 15b, a green light organic light-emitting layer 15g and a cathode reflective layer 16. The transparent electrode layer 14 has three transparent electrodes 141 to 143 respectively pertaining to the OLEDs 11 to 13. In addition, the red light organic light-emitting layer 15r, the blue light organic light-emitting layer 15b and the green light organic light-emitting layer 15g are sequentially disposed on the transparent electrodes 141 to 143, and the cathode reflective layer 16 is disposed on the organic light-emitting layers 15r, 15g and 15g. In addition, in order to increase the lighting efficiency, each of the OLEDs 11 to 13 may further include a hole transporting layer 17 disposed between the transparent electrode layer 14 and the red light organic light-emitting layer 15r, and an electron transporting layer 18 disposed between the green light organic light-emitting layer 15g and the cathode reflective layer 16. Herein, the red light organic light-emitting layer 15r may emit red light, the blue light organic light-emitting layer 15b may emit blue light, the green light organic light-emitting layer 15g may emit green light. The red light, the blue light and the green light are mixed to form white light.
In addition, the OLED 11 further has a red light filter 111 disposed opposite to the transparent electrode 141. The OLED 12 further has a blue light filter 121 disposed opposite to the transparent electrode 142. The OLED 13 further has a green light filter 131 disposed opposite to the transparent electrode 143. Thus, the white light is filtered to form the red light, the blue light and the green light such that the full-color displaying effect can be achieved.
When the OLEDs 11 to 13 are being manufactured, the red light organic light-emitting layer 15r, the blue light organic light-emitting layer 15b and the green light organic light-emitting layer 15g are sequentially deposited on the transparent electrode layer 14. When the red light organic light-emitting layer 15r, the blue light organic light-emitting layer 15b and the green light organic light-emitting layer 15g are disposed on the transparent electrodes 141 to 143, only one metal mask is used in the evaporating procedure. This is very advantageous to the manufacture of the OLEDs 11 to 13 because the yields of the OLEDs 11 to 13 may be influenced if the precision of the metal mask is not very highly controlled and the metal mask cannot be aligned very precisely. However, because each of the OLEDs 11 to 13 needs one filter for filtering, three manufacturing processes have to be performed to form three filters with different colors. Thus, the manufacturing processes are complicated, and the light intensity of each of the OLEDs 11 to 13 is absorbed by the corresponding filter so that the lighting efficiency of the OLED is decreased. In addition, if the light intensity has to be increased, the currents flowing through the OLEDs 11 to 13 have to be increased, thereby shortening the lifetime of each OLED.
Another pixel 1′, as shown in FIG. 1b, is different from the pixel 1 because the pixel 1′ only has a red light organic light-emitting layer 15r′ and a blue light organic light-emitting layer 15b′. The red light emitted from the red light organic light-emitting layer 15r′ and the blue light emitted from the blue light organic light-emitting layer 15b′ are mixed to form the white light, and the red light filter 111, the blue light filter 121 and the green light filter 131 are utilized to filter the white light into the red light, the blue light and the green light respectively so as to achieve the full-color displaying effect. Herein, the distributions of the lighting wavelengths of the red light organic light-emitting layer 15r′ and the blue light organic light-emitting layer 15b′ are respectively wider than those of the red light organic light-emitting layer 15r and the blue light organic light-emitting layer 15b of the pixel 1 so that the good light mixing effect can be achieved. As mentioned hereinabove, this pixel 1′ also has to be manufactured using three manufacturing processes to form the filters with various colors. So, the manufacturing processes are also complicated, the lighting intensity of each OLED is partially absorbed by the filter, the lighting efficiency of each OLED is likewise reduced, and the lifetime of each OLED is shortened.
As shown in FIG. 2, still another conventional pixel 2 includes three OLEDs 21 to 23 each having a transparent electrode layer 24. The transparent electrode layer 24 has three transparent electrodes 241 to 243 respectively pertaining to the OLEDs 21 to 23. In addition, the OLED 21 has a red light organic light-emitting layer 25r disposed on the transparent electrode 241. The OLED 22 has a blue light organic light-emitting layer 25b disposed on the transparent electrode 242. The OLED 23 has a green light organic light-emitting layer 25g disposed on the transparent electrode 243. In addition, the OLEDs 21 to 23 further have a cathode reflective layer 26 disposed on the red light organic light-emitting layer 25r, the blue light organic light-emitting layer 25b and the green light organic light-emitting layer 25g. 
The OLEDs 21 to 23 may respectively output the red light, the blue light and the green light so as to achieve the full-color displaying effect. Because no filter is needed in the OLEDs 21 to 23, the lighting efficiency of each of the OLEDs 21 to 23 may be ensured. However, because the organic light-emitting layers 25r, 25b and 25g are respectively disposed on the transparent electrodes 241 to 243, three metal masks in precise alignment with one another have to be utilized in the evaporating procedure. This is very difficult in the manufacturing processes for high-resolution panel because the yields of the OLEDs 21 to 23 are restricted by the precision and alignment of the metal masks. In addition, the opening of each metal mask is smaller than that of the metal mask used to form the pixel 1 so that the precision has to be higher and the yields of the OLEDs 21 to 23 are thus reduced.
Thus, it is an important subject of the invention to provide an image displaying system having OLEDs, which are manufactured using the reduced number of metal masks, and the reduced number of color filters. Also, the opening of the metal mask is enlarged in order to enhance the yield and the efficiency of each OLED. Thus, the lighting efficiency of the OLED may be enhanced, and the lifetime thereof can be lengthened.